1. Field of the Invention
The present invention concerns a method for production of a superconducting magnetic coil of the type used in a cryomagnet, in particular in a cryomagnet of a magnetic resonance apparatus. Furthermore, the invention concerns a magnetic resonance (MR) apparatus with such a magnetic coil.
2. Description of the Prior Art
Magnetic resonance apparatuses have been firmly established in medical imaging for some decades. Image data of a subject to be examined are acquired utilizing the phenomenon of magnetic resonance of atomic nuclei after the application of various magnetic fields, that are tuned as precisely as possible to one another in terms of their spatial and temporal characteristics.
One of these magnetic fields is a static, strong magnetic field with a field strength of typically 0.2 Tesla to 3 Tesla and more. In most cases this magnetic field is generated by a superconducting magnet. The windings of the superconducting magnetic coils are formed by superconducting wires. When the magnet is energized very large forces (Lorentz forces) that are proportional to the magnetic field act on the superconducting wires. These forces must be absorbed by the winding form (winding body) and the winding itself.
The slightest movements of a wire (which can occur upon energizing, but also afterwards) ultimately lead (by friction and also by the impact of the wire in its new rest position) to an energy feed that is for the most part converted into thermal energy. Due to the severely decreasing heat capacity of all substances at low temperatures, this quickly leads to a temperature increase over the critical temperature of the superconductor. This in turn leads to a quench in which the superconducting state of the magnetic coil collapses and ultimately also the magnetic field generated by the magnetic coil.
Upon the first startup of a superconducting magnet, such quenches (known as “training quenches”) are deliberately provoked. When a wire has moved given an energizing (and thereby has caused a quench), the magnet will accept a higher field strength upon its next energizing (if the wire has occupied its ultimate position). In this manner the wires can be brought into their final position so that their position no longer changes upon subsequent energizings and de-energizings, and quenches are thereby no longer caused.
In order to achieve an optimally large fixing of the wires of the magnetic coil as well as to be able to cool the wires well, the windings of the superconducting coils have conventionally been sealed (potted) with epoxy resin. A number of problems occur with such a production of the magnetic coil.
Pure epoxy resin has a greater thermal coefficient of expansion than the superconducting wire (which is formed essentially of copper), such that upon cooling to the operating temperature of the magnet (4.2 to 10 Kelvin) it exhibits many cracks and thus loses much of its mechanical stability. For example, upon cooling to helium temperature, epoxy resin shrinks by 10 tenths of a percent and copper shrinks by 3 tenths of a percent. In magnetic coils that are sealed with pure epoxy resin, the probability thus increases that they will repeatedly quench upon subsequent energizings. Moreover, the forces acting on the wire (Lorentz forces) are less well absorbed by the wire than in the case of a crack-free sealing material. The forces must primarily be absorbed by a stable (and thus expensive) winding form.
The thermal coefficient of expansion of the epoxy resin can be conformed to that of copper by the admixture of powdered filler material (for example quartz sand), such that the crack formation is at least significantly reduced. The probability that the magnet will quench upon energizing is thereby reduced and the coil form can be retained (mounted) less rigidly because the winding itself can absorb a portion of the Lorentz forces.
If the windings are sealed only with filled epoxy resin after the fabrication thereof, it is possible for the filler material to be filtered out by the narrow gaps in the winding, such that inner regions of the winding are embedded only in epoxy resin without filler material, so (as described above) the stability of the winding is reduced. Upon a subsequent sealing of the winding with a filled epoxy resin, the filler material must therefore exhibit a relatively small grain size (typically one tenth up to one third of the typical wire spacing) and the winding may additionally not include too many layers, so that inner (interior) layers still also receive sufficient filler material. This means that all regions of the winding may not have a sufficient stability, with the disadvantages described above.
Another method with which the filtering out of the filler material is avoided is known as “wet winding”. During winding, filled epoxy resin is applied with an applicator and/or a spatula so that filled epoxy resin is located at each point of the windings. This method has the disadvantage that a contamination of the work space (for example of the winding machine) by dripping epoxy resin occurs. Thickening or curing times of the epoxy resin must additionally be taken into account, which extends the manufacturing process.
Both methods exhibit further disadvantages. The wires must be wound on a stable coil form since the stability is obtained only through the curing of the epoxy resin. For this reason it has conventionally been very time-consuming (and thus uneconomical) to produce a larger self-supporting winding with the wet winding method, since the curing requires a certain time, so the production workflow is significantly lengthened. The described problems lead to increased production costs, so the manufacturing costs for a magnetic resonance apparatus with a superconducting magnetic coil also increase.